To get to Prism i need to use jump point and can i change in mod settings for Prism to sell all BP instead of default group?
Just did a quick test with a new game and the invade option is there on Prism.
Are you on latest version of Nexerelin? (updating won't remove the no-invade tag in existing saves)
The trader always excludes items that don't normally occur as single blueprints, and anything with the NO_DROP or NO_BP_DROP tags, and blueprints that player already knows; this is hardcoded. There's also a list of banned blueprints provided by mods; you could go through each mod and remove/clear the relevant file if you really wanted, but I don't recommend it.
Legio Infernalis has launched an invasion fleet (5-star) on my only planet (I created my own faction) after only a few months in the game. I'm not sure if that is normal. It's not clear to me what to do about it.
Random core worlds weren't activated and I had 0 interaction with Legio Infernalis so far.
Legio starts hostile to pretty much everyone including player, so if you have a juicy colony they might well decide to munch on it. Might be good to do something to make them non-hostile before they invade; handing over a prisoner or AI core should do the job.